The present invention relates generally to ceramic matrix composites, and more particularly to ceramic matrix composites reinforced with combinations of inorganic fibers and whiskers which exhibit enhanced interlaminar shear strength and other desirable properties.
The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, and ceramics has long been practiced. In many references in the literature, whiskers have been characterized as relatively short, single-crystal fibers of small (less than 100 microns) diameter, while fibers are considered to be multicrystalline or amorphous and are generally sufficiently long to be used in woven or otherwise interlocking bundles, tows or cloth. Hence whiskers are typically incorporated as a randomly dispersed phase in a selected glass or ceramic matrix, while fibers are more frequently incorporated in a controlled oriented or interlocking alignment.
The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation in the matrix material.
Whiskers are thought to impart strengthening by a similar mechanism, but load transfer to whiskers by the matrix is more limited due to the limited length and aspect ratio of the whiskers. Theoretically, a whisker which is sufficiently short will not be loaded to the breaking point by the matrix under stress, and therefore full advantage cannot be taken of the high strength of the whiskers.
Among the fibers and whiskers which have been suggested for use as reinforcement for nonmetal matrix materials are silicon carbide, silicon nitride, alumina and carbon whiskers. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of aluminosilicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
A principal objective of whisker reinforcement in ceramic and glass-ceramic materials for high temperature applications is that of increasing the toughness of the material. A toughened ceramic material exhibits improved resistance to cracking failure from flaws sustained in use, offering the possibility of increased fatigue lifetime.
For service at moderate temperatures, fiber and/or whisker-reinforced glasses can be utilized. As noted in U.S. Pat. No. 4,626,515, the addition of fiber reinforcement to glasses such as alkali-free alkaline earth aluminosilicate glasses can result in substantial strengthening, while whisker additions were found effective to enhance the toughness of the glass.
A combination of whiskers and fibers in one of the glasses described in the aforementioned patent, i.e., Corning Code 1723 aluminosilicate glass, resulted in an increase in the microcrack yield stress (MCY) of the glass. The microcrack yield stress of a glass or ceramic material is generally defined as the stress point at which microcrack defects begin to appear in the material. As the stress on the material increases above a certain level, a drop in the elastic modulus of the material is observed, manifested by the appearance of curvature in the normally linear stress-strain curve for the material. It was postulated but not demonstrated in U.S. Pat. No. 4,626,515 that the increase in MCY stress resulting from the combined presence of fibers and whiskers would enhance the transverse strength of the Code 1723 glass composite, thereby reducing fatigue and delamination effects.
In U.S. Pat. No. 4,615,987, a combination of fibers and whiskers was introduced into an anorthrite (CaO-Al.sub.2 O.sub.3 -SiO.sub.2) glass-ceramic matrix and the effects on physical properties determined. Again the MCY stress was raised and similar effects on physical properties were postulated.
For ceramic matrix composites to be utilized in harsh, high-temperature environments, essential characteristics include not only high bending strength and fracture toughness, but also strength properties which are relatively isotropic, i.e., not confined to a single "strong" axis of the composite material. The attainment of such properties normally requires at least some cross-ply lamination of fiber reinforced laminae in the material since, as has been observed, whiskers alone cannot impart the necessary high isotropic modulus of rupture strength to the material. And, in fiber-reinforced ceramic matrix composite of uniaxial fiber orientation, transverse modulus of rupture strengths, i.e. strengths in bending about axes parallel to the fiber direction, are generally at least two orders of magnitude lower than strengths in bending across the fiber direction.
A further problem arising in the development of laminated ceramic matrix composites, including cross-ply laminates, is that of interlaminar shear strength. Stresses applied to the laminated structure in directions parallel to the planes of lamination give rise to shear stresses within interlaminar regions of the composite, which regions are not effectively fiber reinforced. These regions therefore exhibit relatively low strength and provide preferred paths for crack propagation, so that layer separation and delamination of the composite under stress can occur.
It is a principal object of the present invention to provide ceramic matrix composites which exhibit improved transverse modulus of rupture strengths and interlaminar shear strengths.
It is a further object of the invention to provide a method for improving the interlaminar shear strengths and transverse modulus of rupture strengths of fiber-reinforced ceramic matrix composites.
Other objects and advantages of the invention will become apparent from the following description.